In device-to-device (D2D) communications, user equipment (UEs) in proximity of each other can establish a direct radio link to communicate. For this reason, this technology is also known as LTE direct in the literature or as Proximity Service (ProSe) in the 3rd Generation Partnership Project (3GPP) standardization body.
Exploiting a D2D link may allow for very high bit rates, low delays and low power consumption. This makes D2D technology amenable to new types of wireless peer-to-peer services. End-users are increasingly interested in sharing content and/or services with other users in their surroundings, and in establishing peer-to-peer connections to exchange large amounts of data, share interests and applications, and other similar activities. D2D technology also has applications in the fields of national security and public safety. This is because D2D technology may, for example, allow network connectivity to remote UEs in disaster areas via public safety dispatchers (e.g., police, fire brigades, ambulances, etc.) while enjoying the benefits of broadband capabilities.
A basic requirement for network nodes supporting D2D communications is that they should be able to coexist with legacy RAN nodes as the D2D feature is gradually introduced into the existing RAN infrastructure. Consequently, D2D-related functionalities should be implemented taking into account existing LTE infrastructure that can assist D2D UEs in establishing a reliable and secure D2D radio link layer that can coexist with the ordinary cellular link layer. Certainly, D2D communications can also happen in an ad-hoc and autonomous fashion in case the cellular network is damaged, or if D2D-capable UEs do not have network coverage.
D2D-type communication was first standardized under the umbrella of 3GPP in Release 12. In Release 12, the 3GPP standardization body distinguishes between two different types of services: ProSe Direct Communication and ProSe Direct Discovery. In ProSe Direct Communication, UEs in proximity of each other establish a direct user plane connection. In ProSe Direct Discovery, UEs transmit in broadcast and monitor discovery announcements to become aware of the type of content and/or service each UE can share in D2D fashion, as well as the proximity between each other. Enhanced D2D-related features are currently under standardization in 3GPP Release 13. These enhancements include enabling UE-to-Network relay, support for D2D discovery in multiple carriers and out-of-coverage carriers, QoS and priorities aspects.
For ProSe Direct Communication, two different operative modes are specified in 3GPP: mode 1 and mode 2. In mode 1, a UE in RRC_CONNECTED mode requests D2D resources and the eNodeB (eNB) grants them via Physical Downlink Control Channel (PDCCH) (DCIS). This is similar to what happens for cellular uplink (UL) grants. In mode 2, a UE autonomously selects resources for transmission from a pool of available resources that the eNB provides in broadcast via Session Information Block (SIB) 18 (SIB18) for transmissions on carriers other than the Primary Cell (PCell) or via dedicated signaling for transmission on the PCell.
Mode 1 provides a more accurate scheduling from the eNB that can better control the resources actually used for ProSe communication. Mode 1 also provides the UE with a less-interfered resource allocation. Mode 2, on the other hand, requires less implementation complexity and less signaling from the eNB at the expense of possible higher interference. In addition, Mode 1 is only applicable to UEs in RRC_CONNECTED, while mode 2 communications can also be performed by UEs in RRC_IDLE.
Regardless of the mode employed, ProSe communication takes place in the Physical Sidelink Shared Channel (PSSCH) during the so-called SC period. The characteristics of the PSSCH (e.g., length, System Frame Number (SFN) offset, etc.) are signaled by the ProSe discovery carriers. Within the SC period, each PSSCH transmission is preceded by the Scheduling Assignment (SA) period, which takes place in a dedicated Physical Sidelink Control Channel (PSCCH). In the PSCCH, transmitting UEs inform other UEs in the surrounding area on parameters of the following PSSCH data transmission, including Physical Resource Blocks (PRBs), time pattern, and modulation and coding scheme (MCS) parameters. This information helps monitoring UEs correctly perform decoding.
For ProSe Direct Discovery, two different operative modes are specified in 3GPP: type 1 and type 2B. Similar to mode 1 communication in ProSe Direct Communication, in type 2B discovery the eNB provides grants via dedicated Radio Resource Control (RRC) signaling. Similar to mode 2 communication in ProSe Direct Communication, in type 1 discovery a UE autonomously selects discovery resources. These discovery resources are provided either via dedicated signaling for UEs in the PCell and in RRC_CONNECTED, or acquired from SIB19 for UEs that are in RRC_IDLE. More specifically, in type 1 discovery the network provides one or more resource pools. Each resource pool is characterized by a set of PRBs and subframes in which discovery can actually take place. The UE then autonomously selects one of such pools either randomly or on the basis of Reference Signal Received Power (RSRP) measurements related to the different pools. Not all of the resources available in the discovery pool, however, are ultimately used for discovery (i.e., the exact time/frequency resources to use within the pool are selected randomly, and each resource has an equal probability of being selected). In particular, transmission of a discovery Medium Access Control (MAC) Protocol Data Unit (PDU) takes place in one subframe and two PRBs.
In some cases, discovery messages are transmitted in the Physical Downlink Shared Channel (PDSCH) during the discovery period. The PDSCH configuration (e.g., length, SFN offset, etc.) is provided as part of the resource pool configuration. To this end, the subframes within the discovery period that can potentially be used for discovery transmission (or retransmission) are signaled via a bitmap. Within the bitmap, 0 and 1 indicate that this specific subframe is allowed for transmission or is not allowed for transmission, respectively. As described above, the UE has to select one of such subframes and two PRBs for transmission of a discovery message. Each pool may be configured with different bitmap values and bitmap lengths, and can be repeated for a maximum of 5 times within the discovery period. A discovery message can also be retransmitted within the pool using a different puncturing of the encoder according to the principles of soft combining.
A fundamental property of wireless communication is the fact that constructing a device which can send and transmit on the same frequency at the same time is prohibitively expensive. Instead, various duplex schemes are used. One example duplex scheme is Frequency Division Duplex (FDD). FDD uses two carriers separated by a certain number of MHz. This allows a device to transmit on one carrier (the uplink (UL) carrier) and receive on another (the downlink (DL) carrier) at the same time. Thus, a carrier designated for FDD consists of two carriers (one UL and one DL), and are referred to as paired carriers. If the UE knows the frequency of a UL (or DL) carrier, it also knows the corresponding paired DL (or UL) carrier frequency.
Another example duplex scheme is Time Division Duplex (TDD). TDD uses only one carrier, which is divided in time creating slots. Some slots are used for transmission (uplink subframes) and others are used for reception (downlink subframes).
D2D on a FDD carrier by design only uses the uplink carrier. In other words, UEs both transmit and receive on the uplink carrier.
Release 13 does not limit ProSe discovery carrier transmission to the PCell. ProSe discovery transmission can take place in any carrier, including, for example, a Secondary Cell (SCell), a non-serving carrier, a carrier belonging to another eNB (or even to another Public Land Mobile Network (PLMN)), or a preconfigured carrier that is used for public safety operations. Resource configurations for transmitting and monitoring on a non-PCell carrier can be provided in a variety of ways. For example, in cases where TDD mode is used, resource configurations for transmitting and monitoring on a non-PCell carrier can be provided either by the carrier itself via broadcast signaling (e.g., system information such as SIB19) or via a serving cell in the serving eNB with dedicated signaling or broadcast signaling. As another example, in cases where FDD mode is used, resource configurations for transmitting and monitoring on a non-PCell carrier can be provided either by a paired carrier associated with a particular discovery carrier via broadcast signaling (e.g., system information such as SIB19) or via a serving cell in the serving eNB with dedicated signaling or broadcast signaling.
This implies that coordination between different eNBs or different PLMNs should be supported to allow the serving eNB to provide resource configurations for transmitting and/or monitoring on other carriers that do not belong to the serving eNB to which the UE is connected or on which the UE is camping.